back the future trilogy: why they took a long time to make the sequences of the first movie?

if you stop one minute and think a little bit, you can see there's a
delay of four years between the first back to the future release
(1985), and the second and third part of the trilogy (89-90). robert
zemeckis didn't have written the rest of the trilogy history at the
release time of the first movie, or they didn't have means to make the
two sequences earlier ?